Early-Time Estimation of the Thermal Resistance of Flat Specimens — Theoretical Analysis for Pure Conduction in a Homogeneous Material

An examination is made of the validity of estimating steady-state thermal resistance from apparent thermal resistance values obtained from measurements under transient conditions for homogeneous specimens in which heat transfer is entirely by conduction. It is shown that averaging the heat fluxes at the two surfaces of a specimen accelerates convergence to steady-state resistance values if the specimen is initially isothermal at a temperature which differs from its final mean temperature but no improvement is achieved if the specimen mean temperature does not change during testing. It also is shown that the analysis in this paper can be used to predict the thermal resistance from test data taken at early times. The findings presented in this paper must be used with caution since these preliminary analyses do not take into account the effects of radiation through the specimen, the temperature dependence of the thermal properties of the specimen, or the finite thermal resistance and heat capacity of the heat flow meters.